Definitions: “Grayscale topography,” in the context of semiconductor device fabrication, is a defined term referring to the field of variable height three-dimensional nanostructures patterned into a substantially planar surface. The term “variable height” is defined to mean that the depth of features relative to a fiducial plane is a function of displacement within the plane. Thus, height along one axis is a function of lateral displacement transverse to that axis. The term “grayscale topography” encompasses techniques that may, or may not, employ photoresist, whether directly written or exposed via a mask. It is to be noted that, while some literature uses the term “grayscale lithography” in the limited sense of a class of methods for modulating a local energy dose to expose resist thereby producing a three-dimensional (3D) structure during the development step, the term is not so limited in the context of the present description.
“High aspect-ratio,” as the term is used herein and in any appended claims, refers to a structure etched into a substantially planar surface where the ratio of depth relative to the surface with respect to the lateral run over which that depth is varied results in a depth step that exceeds 50%. For example, an etch exceeding 500 nm in depth over no more than 1 μm lateral displacement would constitute a high-aspect-ratio etch.
Where the terms “short,” “intermediate,” and “long” are used to characterize wavelengths, in the present description and in any appended claims, the aforesaid terms are used in a relative sense, which is to say that a short wavelength is shorter than an intermediate wavelength, and that a long wavelength is longer than either a short or an intermediate wavelength.
Similarly, the terms “narrow bandgap,” “medium bandgap,” and “wide bandgap,” as applied, for example, to semiconductor materials, are defined to have relative meaning. That is to say that a narrow bandgap material has a narrower bandgap (i.e., a smaller energy gap between the maximum energy of the valence band and the minimum energy of the conduction band) than a medium bandgap material, and that a wide bandgap material has a wider bandgap than either a narrow bandgap or a medium bandgap material. For avoidance of doubt, it is to be noted that bandgap of silicon is not intended as the standard of comparison for narrow and wide bandgaps, in the current context.
A general reference to the state of the art in microfabrication techniques may be found in Campbell, Fabrication Engineering at the Micro-and Nanoscale, 4th ed., Oxford U. Press (2013), which is incorporated herein by reference. Three-dimensional structures with multiple heights are difficult to achieve using conventional photolithography and etching. Currently, various techniques are employed in the fabrication of semiconductor devices with grayscale topography in particular, and, some of these techniques have entered routine use. Two-photon and multidirectional ultraviolet (UV) photopolymerization of resists are examples of techniques that have been used to realize complex 3D micro- and nano-fabricated patterns. Photo/e-beam complementary grayscale lithography is taught, for example, by Yu et al., “The evaluation of photo/e-beam complementary grayscale lithography for high topography 3D structure,” Proceedings of SPIE 8682, 868212 (2013), incorporated herein by reference.
To fabricate such structures, gray-scale masks can be used, which allow varying amounts of light to pass through and photoactivate a resist applied to the substrate prior to etching. These masks, however, have a limited number of gray levels, are very expensive since the cost scales with the number of levels, and may require several iterative purchases while a given process is perfected. More importantly, gray-scale masks are static and thus cannot easily be modified if the design dimensions or device layout is changed.
As a consequence, focus has been shifted to direct writing techniques which use lasers as the etching tool, rather than gray-scale masks. More complex structures can be created if laser scanning is used, such as the microlens array produced according to the teachings of Chen et al., “Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laser-enhanced local wet etching method,” Opt. Exp., vol. 18, pp. 20334-43 (2010), incorporated herein by reference. Multiple laser beams may be employed to bypass the diffraction limit and obtain sub-micron gratings and nanostructures. Serial laser writing, however, requires precise scanning equipment and software control, and the throughput is relatively low.
Traditionally, photochemical and photoelectrochemical etching have been most often used to improve the material selectivity of particular etching steps within a fabrication process. A survey of the state of the art as of its publication may be found in Kohl, “Photoelectrochemical etching of semiconductors,” IBM J. Res. Dev., vol. 42, pp. 629-37 (1998)(hereinafter, “Kohl (1998)”), which is incorporated herein by reference. Various structures have been fabricated using laser-assisted wet etching, however laser-assisted wet etching has, to date, required proximity masking in order to achieve competitive results.
Although surface topography can be transferred to a semiconductor or other substrate with an etch of appropriate selectivity, it is difficult to transfer the topography of free standing structures with this method or to use the method to perform other types of semiconductor processing, e.g. doping or metallization. Photochemical etching is suited to directly pattern the semiconductor with grayscale topography. When light with sufficient energy is absorbed near the surface of a semiconductor material, minority carriers are generated that can then diffuse to the surface and act as a catalyst in the etching process. As a result, the etch depth for different materials in a given etching solution can be controlled by varying the irradiance, wavelength, or exposure time of the incident light. That much is already known in the art.
Maskless photoelectrochemical etching of 3D structures has been suggested in U.S. Pat. No. 7,433,811 (“Gao”), which is incorporated herein by reference. However, what has eluded achievement to date is one-step etching of multi-level structures by jointly controlling both the temporal and spectral characteristics of the photochemistry-initiating light source as well as the flow of charge carriers in the etched medium, thereby selectively gating aspects of the etching process. That breakthrough is taught in accordance with the present invention.